Re: [PATCH] btrfs: fix rw device counting in __btrfs_free_extra_devids

[Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>]

On 15/7/21 7:55 pm, Nikolay Borisov wrote:
> 
> 
> On 15.07.21 г. 13:34, Desmond Cheong Zhi Xi wrote:
>> Syzbot reports a warning in close_fs_devices that happens because
>> fs_devices->rw_devices is not 0 after calling btrfs_close_one_device
>> on each device.
>>
>> This happens when a writeable device is removed in
>> __btrfs_free_extra_devids, but the rw device count is not decremented
>> accordingly. So when close_fs_devices is called, the removed device is
>> still counted and we get an off by 1 error.
>>
>> Here is one call trace that was observed:
>>    btrfs_mount_root():
>>      btrfs_scan_one_device():
>>        device_list_add();   <---------------- device added
>>      btrfs_open_devices():
>>        open_fs_devices():
>>          btrfs_open_one_device();   <-------- rw device count ++
>>      btrfs_fill_super():
>>        open_ctree():
>>          btrfs_free_extra_devids():
>> 	  __btrfs_free_extra_devids();  <--- device removed
>> 	  fail_tree_roots:
>> 	    btrfs_close_devices():
>> 	      close_fs_devices();   <------- rw device count off by 1
>>
>> Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device")
>> Reported-by: syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
>> Tested-by: syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> 
> Is there a reliable reproducer from syzbot? Can this be turned into an
> xfstest?
> 

Syzbot has some reliable reproducers here:
https://syzkaller.appspot.com/bug?id=113d9a01cbe0af3e291633ba7a7a3e983b86c3c0

Seems like it constructs two images in-memory then mounts them. I'm not 
sure if that's amenable to be converted into an xfstest?

>> ---
>>   fs/btrfs/volumes.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index 807502cd6510..916c25371658 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -1078,6 +1078,7 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
>>   		if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
>>   			list_del_init(&device->dev_alloc_list);
>>   			clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
>> +			fs_devices->rw_devices--;
>>   		}
>>   		list_del_init(&device->dev_list);
>>   		fs_devices->num_devices--;
>>